I was working on flash file modification project recently and had to make use of drop down (ComboBox) control present in the flash 2004 UI Components. All was set well and when trying to execute the flash file, the component did not work or rather there was no component but just an empty rectangle in the place. After doing some tussling with the flash settings, I remembered that the flash file was created with an older version, and when I had opened it in flash 2004 pro, it had not set the action script verion to 2 (it was in version 1).
So after reseting the action script verion to 2 in the publish settings panel, the control showed up properly.